WASHINGTON -- Scientists have mapped how a group of fearsome dinosaurs evolved and shrank to the likes of robins and hummingbirds.

Comparing fossils of 120 different species and 1,500 skeletal features, especially thigh bones, researchers constructed a detailed family tree for the class of two-legged meat-eaters called theropods. That suborder of dinosaurs survives to this day as birds, they said.

The steady downsizing and evolution of the theropods was detailed in the journal Science on Thursday.

"They just kept on shrinking and shrinking and shrinking for about 50 million years," said study author Michael S.Y. Lee of the University of Adelaide in Australia. He called them "shape-shifters."

Lee and colleagues created a dinosaur version of the iconic ape-to-man drawing of human evolution. In this version, the lumbering large dinosaurs shrink, getting more feathery and big-chested, until they are the earliest version of birds.

For a couple of decades scientists have linked birds to this family of dinosaurs because they shared hollow bones, wishbones, feathers and other characteristics. But the Lee study gives the best picture of how steady and unusual theropod evolution was. The skeletons of theropods changed four times faster than other types of dinosaurs, the study said.

A few members of that dinosaur family did not shrink, including T. rex, which is more of a distant cousin to birds than a direct ancestor, Lee said.

He said he and colleagues were surprised by just how consistently the theropods shrank over evolutionary time, while other types of dinosaurs showed ups and downs in body size.

The first theropods were large, weighing about 600 pounds. They roamed about 220 million to 230 million years ago.

Then, about 200 million years ago, when some of the creatures weighed about 360 pounds, the shrinking became faster and more prolonged, the study said. In just 25 million years, the beasts were slimmed down to barely 100 pounds.

By 167 million years ago, six-pound paravians, more direct ancestor of birds, were around. And 163 million years ago the first birds, weighing less than 2 pounds, probably came on the scene, the study said.

Paul Sereno, a dinosaur researcher at the University of Chicago who wasn't part of this study, praised Lee's work as innovative.

The steady size reduction shows "something very strange going on," Sereno said. "This is key to what went on at the origin of birds."